The demand for high-capacity secondary batteries is growing accompanying the reduced size of electronic devices. Nonaqueous secondary batteries, and particularly lithium ion secondary batteries, which demonstrate high energy density in comparison with nickel-cadmium batteries or nickel-hydrogen batteries, are attracting particular attention. Lithium ion secondary batteries, which are composed of positive and negative electrodes capable of occluding and releasing lithium ions and a nonaqueous electrolytic solution obtained by dissolving a lithium salt such as LiPF6 or LiBF4, have been deployed not only in conventional laptop personal computers, mobile communications devices, portable cameras and handheld game consoles, but also in power tools and electric automobiles, thus resulting in a growing need for higher capacities, faster charging and discharging characteristics and higher cycling characteristics in lithium ion secondary batteries accompanying their deployment in such applications.
Although various types of materials have been proposed for the negative electrode material of these batteries, natural graphite, artificial graphite obtained by graphitization of coke and the like, graphitized mesophase pitch and graphitic carbon materials such as graphitized carbon fiber are used at present due to their high capacity and the superior flatness of their discharge potential.
In recent years, studies have been conducted on the application of materials having high theoretical capacity, and particularly metal particle negative electrodes, with the aim of further increasing capacity.
For example, Patent Documents 1 and 2 propose methods for producing composite Si-graphite particles by firing a mixture of fine particles of an Si compound, graphite and a carbonaceous precursor in the form of pitch and the like.
Patent Document 3 proposes composite Si-graphite particles obtained by compounding Si fine particles with a carbonaceous material so that the Si fine particles are unevenly distributed on the surface of spherical natural graphite.
Patent Document 4 proposes composite graphite particles having as main components thereof metal capable of alloying with Li, flake graphite and a carbonaceous material, wherein the metal is held between a plurality of flake graphite layers, and specifically discloses composite Si-graphite particles.
Patent Document 5 proposes composite graphite particles composed of a granulated body obtained by crushing and granulating a mixture of a graphite raw material and metal powder in a high-speed airflow, wherein a portion of the graphite serving as raw material is crushed and the graphite raw material and its crushed product aggregate to form a laminated structure in which metal powder is present in a dispersed state internally and on the surface thereof, and specifically discloses composite Si-graphite particles.
Patent Document 6 discloses composite Si-graphite particles composed of roughly spherical particles having carbon microprotrusions on the surface thereof obtained by granulating and spheroidizing a mixture of void forming agents selected from vein and/or flake natural graphite, fine particles of an Si compound, carbon black, polyvinyl alcohol, polyethylene glycol, polycarbosilane, polyacrylic acid and cellulose-based polymers, and impregnating and coating the resulting spherical granulation product with a mixture of a carbon precursor and carbon black followed by firing.
Patent Document 7 discloses composite Si-graphite particles of a form in which Si particles are sandwiched between flake graphite, and are obtained by applying compressive force and shearing force to a mixture of Si particles, flake graphite and a solid non-graphitic carbon raw material at a temperature equal to or higher than the softening point of the non-graphitic carbon raw material to prepare intermediate composite particles followed by heat treatment.
Patent Document 8 discloses composite Si-graphite particles having a structure in which graphite is folded by mixing Si particles and flake graphite and subjecting to spheroidizing treatment.